Field of the Invention
The present invention relates to display technology. More specifically, the present invention relates to digital backplanes that control light modulating elements, spatial light modulators and light sources.
Discussion of Related Art
Micro-displays typically include light modulating backplane and a light modulating unit or a light emitting unit. Light modulating units include such technologies as liquid crystal on silicon (LCOS) and digital micro mirrors devices (DMD). Light emitting units include technologies such as Organic light emitting diodes (OLED). The technology used in such micro displays can also be used to make larger display units.
FIGS. 1A and 1B illustrate a small portion of a conventional LCOS display 100. Specifically, FIG. 1B only shows 24 pixels of LCOS display 100. Generally, a LCOS display would have thousands of pixels. FIG. 1A is a cross sectional view of display 100 along the A A′ cut shown in FIG. 1B. FIG. 1B shows only one layer of LCOS display 100. Specifically, FIG. 1B shows the top of the reflective pixel electrodes of LCOS display 100.
In FIG. 1A, a substrate 110 supports the active backplane region 120 which includes pixel control circuits PCC_1_1, PCC_2_1, PCC_3_1, PCC_4_1, PCC_5_1, and PCC_6_1 and pixel electrodes PE_1_1, PE_2_1, PE_3_1, PE_4_1, PE_5_1, and PE_6_1. The pixel electrodes are located above the pixel control circuits. Each pixel electrode PE_X_Y is coupled to and controlled by pixel control circuit PCC_X_Y. Thus, pixel electrode PE_1_1 is coupled to and controlled by pixel control circuit PCC_1_1. Similarly, electrodes PE_2_1, PE_3_1, PE_4_1, PE_5_1, and PE_6_1 are coupled to and controlled by pixel control circuits PCC_2_1, PCC_3_1, PCC_4_1, PCC_5_1, and PCC_6_1, respectively. For LCOS display 100, the pixel electrodes are made of a reflective conductor to reflect incoming light as explained below. As shown in FIG. 1B, the polarized electrodes are arranged in a rectangular matrix. For clarity the pixel electrodes are PE_X_Y, where X refers to the column location of the pixel electrode and Y refers to the row location of the pixel electrode.
Active backplane region 120 also includes various, logic circuits to support the operation of the pixel control circuits. For clarity these logic circuits are omitted in the Figures because the omitted logic circuits, which are well known in the art, are not an integral aspect of the present invention. Substrate 110, the pixel control circuits, the pixel electrodes and the omitted logic circuits form a light modulating backplane 100b. In addition, a transparent passivation layer (not shown in FIGS. 1A and 1B) covers the pixel electrodes. An example of a light modulating backplane is described in U.S. Pat. No. 7,071,908, entitled “Digital Backplane” by Guttag et al., which is included herein by reference. Another example of a light modulating backplane is described in U.S. Pat. No. 8,605,015 entitled “Spatial Light Modulator with Masking Comparators” by Guttag et al., which is incorporated herein by reference.
The light modulating unit 100a of LCOS display 100 includes a liquid crystal layer 130, an alignment layer 140, a transparent common electrode layer 150, and a protective glass layer 160. Protective glass layer 160 protects the rest of LCOS display 100 but typically does not manipulate incoming or reflected light. Transparent common electrode layer 150 works with the pixel electrodes to manipulate the liquid crystals in liquid crystal layer 130. Alignment layer 140 aligns the liquid crystals in liquid crystal layer 130 to properly manipulate incoming and reflected light. Liquid crystal layer 130 contains liquid crystals that are controlled by the pixel electrodes to selectively pass incoming polarized light through liquid crystal layer 130. Specifically, when a pixel electrode is charged to an “active state” by the corresponding pixel control circuit polarized light can pass through the area of liquid crystal layer 130 above the pixel electrode and be reflected back by the pixel electrode. However, if the pixel electrode is in an inactive state polarized light is blocked in the area of liquid crystal layer 130 above the pixel electrode. Pulse width modulation is used to create different contrast levels. For color displays, color filters can be included in the light modulating unit or field sequential color schemes (i.e. rapidly cycling through three different colored light sources) can be used.
The light modulating backplane of conventional LCOS displays are made using a LCOS process on top of structures made with standard CMOS process. Specifically, the pixel control circuits (and supporting circuits) are made using standard CMOS process while the pixel electrodes are made using the LCOS process. FIG. 2 is a cross sectional view of display 100 along the A A′ cut shown in FIG. 1B. However FIG. 2 only shows the portion of the A A′ cut that includes pixel electrodes PE_1_1 and PE_1_2. FIG. 2 shows a very simplified diagram of the various metal layers in a portion active backplane region 120 of light modulating backplane 100b of LCOS display 100. Specifically for light modulating backplane 100b, 4 metal layers (typically named M1-M4) are used in the CMOS process. However other backplanes can use more or fewer metal layers. In general advanced CMOS processes use copper for the metal layers. In addition a global metal layer (named GM), is used for signals used across the entire display such as power lines, ground lines, and clock lines. In general global metal layer GM is very thick (e.g. 700 to 1300 nanometers) and is made using an additional aluminum layer for light modulating backplanes. An LCOS process is used to fabricate the pixel electrodes and the connection between the pixel electrodes and the metal layers of the pixel control circuits, which were made with the CMOS process. Aluminum is used in conventional LCOS displays for the pixel electrodes.
In FIG. 2, the various conductors and vias only illustrate the relative locations of the metal layers and do not actually form working circuits. Many details and various processing layer, which are well known in the art are omitted for clarity. For additional clarity, aluminum conductors are drawn with light shading, copper conductors are drawn with medium shading, tungsten conductors (vias) are drawn with dark shading, and transparent layers are drawn with no shading. Metal layer M1 includes copper conductor M1_1, M1_2, and M1_3. Copper conductor M1_2 is orthogonal to copper conductors M1_1 and M1_3 and thus appears very short as compared to copper conductors M1_1 and M1_3. Metal layer M2 includes copper conductor M2_1, M2_2, M2_3, and M2_4. Copper conductor M2_1 is coupled to copper conductor M1_1 by a via V1. Copper conductor M2_4 is coupled to copper conductor M1_3 by a via V2.
Metal layer M3 includes copper conductor M3_1 and M3_2. Copper Conductor M3_1 is coupled to copper conductor M2_2 by a via V3. Metal layer M4 includes copper conductor M4_1, M4_2, M4_3, and M4_4. Copper conductor M4_2 is coupled to copper conductor M3_1 by a via V5. Copper conductor M4_4 is coupled to copper conductor M3_2 by a via V4.
Metal conductors M4_2 is also coupled to pixel electrode PE_1_1 by a LCOS via LV_1, a LCOS Stud LS_1, which is formed from the global metal layer, and a LCOS via LV_2. Specifically, LCOS via LV_1 couples copper conductor M4_2 to LCOS stud LS_1, which is part of global metal layer GM. LCOS via LV_2 couples LCOS stud LS_1 to pixel electrode PE_1_1. Because copper electrodes M4_2, M3_1, and M2_2 are coupled to pixel electrode PE_1_1, copper electrodes M4_2, M3_1, and M2_2 are components of pixel control circuit PCC_1_1 (See FIG. 1A).
Metal conductors M4_4 is coupled to pixel electrode PE_2_1 by a LCOS via LV_3, a LCOS Stud LS_2, and a LCOS via LV_4. Specifically, LCOS via LV3 couples copper conductor M4_4 to LCOS stud LS_2, which is part of global metal layer GM. LCOS via LV4 couples LCOS stud LS_2 to pixel electrode PE_2_1. Because copper electrodes M4_4 and M3_2 are coupled to pixel electrode PE_2_1, copper electrodes M4_4 and M3_2 are components of pixel control circuit PCC_2_1 (See FIG. 1A). Global metal layer also includes aluminum conductors GM_1, GM_2 and GM_3.
As stated above, pixel electrodes PE_1_1 and PE_2_1 are formed with aluminum for among other reasons, the high reflectivity of aluminum and the stability of aluminum. In LCOS displays the global metal layer GM can be made using either copper or Aluminum. In LCOS display 100, global metal layer GM is an aluminum layer. LCOS vias LV_1, LV_2, LV_3, and LV_4 are made using tungsten, which can provide good electrical contacts with both copper and aluminum. A transparent passivation layer 210 covers the top of the light modulating backplane. In a light modulating backplane in accordance with one embodiment of the present invention, the pixel electrodes have a thickness of 260 nanometers, a width of 6.2 μm and a length of 6.2 μm. The transparent passivation layer is silicon dioxide with a thickness of 75 nanometers, Global metal layer GM has a thickness between 700 and 1300 nanometers, and metal layers M1-M4 have a thickness between 70 and 500 nanometers.
Very few microchip fabrication plants (hereinafter fabs or fab) are configured to manufacture the light modulating backplane of a LCOS display primarily due to the difficulties and additional costs of connecting the aluminum pixel electrodes to the copper metal layers of the active backplane region. Thus these few fabs can charge excessive prices to manufacture the light modulating backplanes of LCOS displays. Hence there is a need for a light modulating backplane of a LCOS display that can be manufactured using CMOS processes.